Zoom lens

ABSTRACT

A zoom lens of the rear focus type is disclosed, whose back focal distance is increased to accommodate a three-color separation prism and other optical elements, comprising, in order from an object side to an image side, a first lens unit of positive refractive power, a second lens unit of negative refractive power, a third lens unit of positive refractive power and a fourth lens unit of positive refractive power, the second lens unit and the fourth lens unit being moved to effect zooming and the fourth lens unit being moved to effect focusing, wherein the third lens unit has a negative lens disposed closest to the object side and having a concave surface facing the object side and has a positive lens disposed closest to the image side, and a lens surface on the image side of the positive lens has a refractive power which is stronger than that of a lens surface on the object side of the positive lens.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to zoom lenses of the rear focus type and, moreparticularly, to zoom lenses of the rear focus type with a colorseparation prism in the space between the rear vertex and the CCD. Stillmore particularly, this invention relates to high range, large relativeaperture zoom lenses of the rear focus type which provide, despitesecuring of such a long back focal distance, increase of the zoom ratiowhile still permitting the diameter of the front lens members to beminimized.

2. Description of Related Art

Recently, home video cameras of ever smaller size and lighter weighthave been developed. Along with this development, a remarkable advanceis seen even in decreasing the bulk and size of its taking zoom lens. Inparticular, efforts are being devoted to shortening of the total lengthof the entire lens system, reduction of the diameter of the front lensmembers and simplification of the form and the construction andarrangement of the constituent parts.

To attain these ends, one means is to move a lens unit other than thefront or first lens unit to effect focusing. Such a zoom lens is knownas the so-called “rear focus type”.

In general, the rear focus type of zoom lens has many advantages overthe type which performs focusing by moving the front lens unit. Forexample, it becomes easier to improve the compact form of the entirelens system. Close-up photography, particularly supershort focusing,becomes possible. Further, since the focusing lens unit is of small sizeand light weight, because the required driving torque for moving thefocusing lens unit is reduced, rapid focus adjustment can be carriedout.

Such a rear focus type of zoom lens is disclosed in, for example,Japanese Laid-Open Patent Applications Nos. Sho 62-206516, Sho 62-215225and Sho 62-24213, in which the zoom lens comprises, in order from anobject side, a positive first lens unit, a negative second lens unit, apositive third lens unit and a positive fourth lens unit, the secondlens unit being axially moved to vary the focal length, and the fourthlens unit being axially moved to compensate for the image shift withzooming and also to effect focusing.

Also, in Japanese Laid-Open Patent Applications Nos. Hei 4-43311, Hei4-153615, Hei 5-19165, Hei 5-27167 and Hei 5-60973, there are disclosedexamples of the fourth lens unit consisting of one or two positivelenses. Also, in Japanese Laid-Open Patent Application No. Hei 5-60974,there is disclosed a zoom lens whose fourth lens unit is constructedwith positive and negative lenses, totaling two lenses.

Further, in Japanese Laid-Open Patent Applications Nos. Sho 55-62419,Sho 62-24213, Sho 62-215225, Sho 56-114920, Hei 3-200113, Hei 4-242707,Hei 4-343313 and Hei 5-297275, there are disclosed, in theirembodiments, zoom lenses in which the third and fourth lens units eachare constructed with a positive lens and a negative lens, totaling twolenses.

Another recent trend in the art of video cameras has come along withenhancement (digitization) of the performance of video decks. To measureup to this, the image quality must ever more advance. One method ofattaining the high image quality is to split the image by using a colorseparation optical system. Lenses that are suited to be used with thissystem have been proposed as disclosed in Japanese Laid-Open PatentApplications Nos. Hei 5-72474, Hei 6-51199, Hei 6-337353, Hei 6-347697,Hei 7-199069 and Hei 7-270684, etc.

As mentioned above, in general, for the zoom lenses, in view ofachieving valuable reduction of the diameter of the front lens memberswith the size of the entire system at a minimum, the so-called “rearfocus” configuration is more suitable than that of focusing by the firstlens unit.

In the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei4-026811 and Hei 4-88309, however, their configurations are hardlyamenable to dispose the color separation prism.

In the zoom lenses disclosed in the above-mentioned Japanese Laid-OpenPatent Applications Nos. Hei 4-43311, Hei 4-153615, Hei 5-19165, Hei5-27167 and Hei 5-60973, the zoom ratio is 6 to 8 or thereabout. Forzoom lenses of higher ranges than this, the variation of chromaticaberrations with zooming would becomes too large to correct well. It is,therefore, difficult to assure maintenance of sufficient opticalperformance. Even the examples disclosed in the above-mentioned JapaneseLaid-Open Patent Application No. Hei 5-60974, too, have a zoom ratio of8, so that no sufficient increase of the range is achieved.

Further, in the examples disclosed in the above-mentioned JapaneseLaid-Open Patent Applications Nos. Sho 55-62419, Sho 56-114920 and Hei3-200113, either the first lens unit or the third lens unit, too, ismade to move during zooming. This leads to increase the complexity ofthe operating mechanism. These zoom lenses are, therefore, not suited toachieve improvements of the compact form. In the examples disclosed inthe above-mentioned Japanese Laid-Open Patent Applications Nos. Hei4-242707, Hei 4-343313 and Hei 5-297275, construction and arrangementare made such that the third lens unit has a large air space. Inaddition, a negative lens included in the third lens unit is relativelyweak in refractive power. In application to high range zoom lenses,therefore, this does not become a type that assures sufficientcorrection of the chromatic aberrations the third lens unit wouldproduce. Furthermore, in the example disclosed in the above-mentionedJapanese Laid-Open Patent Application No. Hei 5-297275, a negativemeniscus lens included in the third lens unit is made to have a strongconcave curvature toward the image side. This is advantageous forobtaining the telephoto form, but does not favor the negative lens totreat the flare component of higher orders the positive lens produces.Such a type is, therefore, disadvantageous to be used in the largerelative aperture, high range zoom lenses.

In the above-mentioned Japanese Laid-Open Patent Applications Nos. Hei5-72474, Hei 6-51199, Hei 6-337353, Hei 6-347697, Hei 7-199069 and Hei7-270684, any of their embodiments has as low a zoom ratio as 10 to 12.Thus, the sufficient increase of the range is also not achieved.

BRIEF SUMMARY OF THE INVENTION

An object of the invention is to overcome the above-described drawbacksof the conventional examples and to improve, in particular, the zoomlens disclosed in the above-mentioned Japanese Laid-Open PatentApplication No. Hei 7-270684. The space an optical element such as colorseparation prism and another optical element which has an aim to protectthe zoom lens are to occupy, is secured sufficiently at the back focaldistance. Despite this, good optical performance is maintainedthroughout the entire zooming range and throughout the entire focusingrange. Another object of the invention is to provide a zoom lens of therear focus type which has a large relative aperture and whose range isincreased to 16 or thereabout. A concomitant object of the invention isto provide a video camera to which such a zoom lens is releasablyattached.

To attain the above objects, in accordance with one aspect of theinvention, there is provided a zoom lens of the rear focus type,comprising, in order from an object side to an image side, a first lensunit of positive refractive power, a second lens unit of negativerefractive power, a third lens unit of positive refractive power and afourth lens unit of positive refractive power, the second and fourthlens units being moved to effect zooming, and the fourth lens unit beingmoved to effect focusing, wherein the third lens unit has a positivelens disposed closest to the image side, and a lens surface on the imageside of the positive lens has a refractive power which is stronger thanthat of a lens surface on the object side of the positive lens. Further,the third lens unit has a negative lens disposed closest to the objectside and having a concave surface facing the object side.

In more detail, to make approximately afocal a light beam which would bediverged by the third lens unit, the third lens unit is provided withthe positive lens disposed closest to the image side. The form of thepositive lens is then so specified that the configuration approaches theretro-focus type, thereby bringing the principal point of the third lensunit away from the second lens unit. As the principal points of thesecond and third lens units open wider, the height of incidence of theon-axial ray on the third lens unit becomes higher. This leads to apossibility of making longer the required focal length of the fourthlens unit for the predetermined values of the focal length of the entiresystem. In such a manner, the working distance or the back focaldistance is much increased. That is, because the light beam exiting fromthe third lens unit is almost afocal, the back focal distance ascalculated in the principal point system becomes almost the same as thefocal length of the fourth lens unit. Under the condition that the focallength of the entire system is fixed, the focal length of the fourthlens unit is made longer. To this purpose, therefore, it isunderstandable from FIG. 19 that all what is need to do is only toincrease the height of incidence “h” of the on-axial light ray on thethird lens unit.

Further, according to the invention, the following conditions aresatisfied:

1.0<|R _(31r) /R _(32r)|<5.0

1.5<f ₃ /f ₃₂<5.0

where R_(31r) and R_(32r) are radii of curvature of the lens surfaces onthe object side and on the image side of the positive lens,respectively, and f₃₂ and f₃ are focal lengths of the positive lens andthe third lens unit, respectively.

When these conditions are satisfied, the requirements of securing theback focal distance and of correcting aberrations are fulfilled at once.

In particular, it is preferable that the positive lens of the third lensunit is a cemented lens composed of a positive lens and a negative lens.

These and other objects and features of the invention will becomeapparent from the following detailed description of the preferredembodiments thereof taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING

FIG. 1 is a lens block diagram of a numerical example 1 of theinvention.

FIG. 2 is a lens block diagram of a numerical example 2 of theinvention.

FIG. 3 is a lens block diagram of a numerical example 3 of theinvention.

FIG. 4 is a lens block diagram of a numerical example 4 of theinvention.

FIG. 5 is a lens block diagram of a numerical example 5 of theinvention.

FIG. 6 is a lens block diagram of a numerical example 6 of theinvention.

FIG. 7 is a lens block diagram of a numerical example 7 of theinvention.

FIG. 8 is a lens block diagram of a numerical example 8 of theinvention.

FIG. 9 is a lens block diagram of a numerical example 9 of theinvention.

FIGS. 10A-1 to 10A-4, 10B-1 to 10B-4 and 10C-1 to 10C-4 are graphicrepresentations of the aberrations of the numerical example 1 of theinvention.

FIGS. 11A-1 to 11A-4, 11B-1 to 11B-4 and 11C-1 to 11C-4 are graphicrepresentations of the aberrations of the numerical example 2 of theinvention.

FIGS. 12A-1 to 12A-4, 12B-1 to 12B-4 and 12C-1 to 12C-4 are graphicrepresentations of the aberrations of the numerical example 3 of theinvention.

FIGS. 13A-1 to 13A-4, 13B-1 to 13B-4 and 13C-1 to 13C-4 are graphicrepresentations of the aberrations of the numerical example 4 of theinvention.

FIGS. 14A-1 to 14A-4, 14B-1 to 14B-4 and 14C-1 to 14C-4 are graphicrepresentations of the aberrations of the numerical example 5 of theinvention.

FIGS. 15A-1 to 15A-4, 15B-1 to 15B-4 and 15C-1 to 15C-4 are graphicrepresentations of the aberrations of the numerical example 6 of theinvention.

FIGS. 16A-1 to 16A-4, 16B-1 to 16B-4 and 16C-1 to 16C-4 are graphicrepresentations of the aberrations of the numerical example 7 of theinvention.

FIGS. 17A-1 to 17A-4, 17B-1 to 17B-4 and 17C-1 to 17C-4 are graphicrepresentations of the aberrations of the numerical example 8 of theinvention.

FIGS. 18A-1 to 18A-4, 18B-1 to 18B-4 and 18C-1 to 18C-4 are graphicrepresentations of the aberrations of the numerical example 9 of theinvention.

FIG. 19 is a diagram of geometry for explaining the principle of thezoom lens of the invention.

FIG. 20 is a lens block diagram of a numerical example 10 of theinvention.

FIG. 21 is a lens block diagram of a numerical example 11 of theinvention.

FIG. 22 is a lens block diagram of a numerical example 12 of theinvention.

FIGS. 23-1 to 23-4 are graphic representations of the aberrations of thenumerical example 10 of the invention in the wide-angle end.

FIGS. 24-1 to 24-4 are graphic representations of the aberrations of thenumerical example 10 of the invention in a middle focal length position.

FIGS. 25-1 to 25-4 are graphic representations of the aberrations of thenumerical example 10 of the invention in the telephoto end.

FIGS. 26-1 to 26-4 are graphic representations of the aberrations of thenumerical example 11 of the invention in the wide-angle end.

FIGS. 27-1 to 27-4 are graphic representations of the aberrations of thenumerical example 11 of the invention in a middle focal length position.

FIGS. 28-1 to 28-4 are graphic representations of the aberrations of thenumerical example 11 of the invention in the telephoto end.

FIGS. 29-1 to 29-4 are graphic representations of the aberrations of thenumerical example 12 of the invention in the wide-angle end.

FIGS. 30-1 to 30-4 are graphic representations of the aberrations of thenumerical example 12 of the invention in a middle focal length position.

FIGS. 31-1 to 31-4 are graphic representations of the aberrations of thenumerical example 12 of the invention in the telephoto end.

In the aberration curves, IP stands for the image plane, AM for themeridional image focus, AS for the sagittal image focus, d for thespectral d-line, and g for the spectral g-line.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, preferred embodiments of the invention will be described indetail with reference to the drawings.

FIG. 1 through FIG. 9 are longitudinal section views of numericalexamples 1 to 9 of zoom lenses of the rear focus type of the invention,respectively. The aberrations of the numerical examples 1 to 9 are shownin FIGS. 10A-1 to 10A-4, 10B-1 to 10B-4 and 10C-1 to 10C-4 through FIGS.18A-1 to 18A-4, 18B-1 to 18B-4 and 18C-1 to 18C-4, respectively. Of theaberration curves, the ones whose figure numbers are suffixed A are inthe wide-angle end, the ones whose figure numbers are suffixed B are inthe middle position and the ones whose figure numbers are suffixed C arein the telephoto end.

In the block diagrams, reference character L1 denotes the first lensunit of positive refractive power, reference character L2 denotes thesecond lens unit of negative refractive power, reference character L3denotes the third lens unit of positive refractive power and referencecharacter L4 denotes the fourth lens unit of positive refractive power.An aperture stop SP is disposed just in front of the third lens unit L3.Reference character GA denotes a glass plate for protecting the zoomlens. Reference character GB is a glass block such as a color separationprism, a face plate for the CCD or a low-pass filter. Parts from thefirst lens unit L1 to the protection glass plate GA constitute a zoomlens part, which is releasably attached to a camera body part through amount member C. Therefore, the glass block GB and the subsequent partsare included in the camera body part.

In the present embodiment, during zooming from the wide-angle end to thetelephoto end, the second lens unit moves axially toward the image sideas shown by the arrow and the fourth lens unit axially moves tocompensate for the shift of an image plane. For focusing purposes, thefourth lens unit is also made axially movable. That is, the rearfocusing method is employed. In particular, as shown by the curved lines4 a and 4 b in FIG. 1, the fourth lens unit is made to move, whiledepicting a locus convex toward the object side, as zooming from thewide-angle end to the telephoto end, thereby assuring efficientutilization of the space between the third and fourth lens units. Ashortening of the total length of the entire lens system is thusachieved advantageously. The solid line curve 4a and the dashed linecurve 4b represent the required loci of motion for compensating for theimage shift over the entire zooming range with an object at infinity andat a minimum distance, respectively. It is to be noted that the firstand third lens units remain stationary during zooming and duringfocusing.

In the numerical examples 1 to 3 and 9, the third lens unit isconstructed with a cemented lens composed of negative and positivelenses and having a negative refractive power and a cemented lenscomposed of positive and negative lenses and having a positiverefractive power. As a whole, these cemented lenses constitute apositive lens unit of the retro-focus type. Moreover, the cemented lensof negative refractive power has a concave surface facing the objectside, thus giving the role of bringing the principal point of the thirdlens unit away from the second lens unit. So, this contributes to anincrease of the back focal distance. In particular, the lens surface onthe object side of the cemented lens of negative refractive power isgiven a stronger negative refractive power (shorter radius of curvature)than that of the lens surface on the image side, so that the principalpoint takes its place closer to the image plane than was heretoforepossible.

Meanwhile, a lens surface on the image side of the cemented lens ofpositive refractive power has a refractive power which is stronger(shorter in the radius of curvature) than that of a lens surface on theobject side of the cemented lens of positive refractive power. Thiscemented lens, too, bears a similar role in bringing the principal pointof the third lens unit away from the second lens unit. So, it assists inincreasing the focal length of the fourth lens unit, contributing to anincrease of the back focal distance.

Similarly, in the numerical examples 4 to 8, the third lens unit isconstructed with a negative single lens and a positive single lens. As awhole, they constitute a positive lens unit of the retro-focus type.Further, a lens surface on the object side of the negative single lensis strong in concave curvature toward the object side as it bears therole of bringing the principal point of the third lens unit away fromthe second lens unit. So, it assists in increasing the focal length ofthe fourth lens unit, contributing to an increase of the back focaldistance.

Meanwhile, a lens surface on the image side of the positive single lenshas a stronger refractive power (shorter in the radius of curvature)than that of a lens surface on the object side of the positive singlelens. This lens bears a role of bringing the principal point of thethird lens unit away from the second lens unit. So, it assists inincreasing the focal length of the fourth lens unit, contributing to anincrease of the back focal distance.

It will be appreciated that in the embodiment of the invention, thethird lens unit is provided with a positive lens disposed closest to theimage side, wherein a lens surface on the image side of the positivelens is made convex toward the image side, thereby bringing theprincipal point to a more rear position. Thus, to allow the colorseparation prism to be disposed behind the zoom lens, the increase ofthe back focal distance is thus secured.

Although the foregoing features suffice for providing the zoom lens thatsufficiently secures the long back focal distance and has a high range,in order to achieve further improvements, it is preferable to satisfyone of the following conditions:

(i) Letting a distance, when reduced to air, from the last lens surfaceof the zoom lens to an image plane in the wide-angle end with an objectat infinity be denoted by BF, and a focal length, a full apertureF-number and a semiangle of field in the wide-angle end of the entirezoom lens be denoted by f_(W), F_(NW), and ω, respectively, thefollowing condition is satisfied:

8.1<BF×{square root over (F_(NW)+L )}/( f _(W)×tan ω)<13.3  (1).

When the lower limit is exceeded, as this means that the zoom lens istoo faster, spherical aberrations and come of higher orders areproduced, which are difficult to correct.

When the F-number is darker than the upper limit, the axial light beambecomes smaller in diameter.

By this, it becomes possible to reduce the size of the color separationprism in the space between the last lens surface of the zoom lens andthe image plane. In other words, despite the fact that there is no needto elongate the back focal distance, the back focal length must be madelonger. Thus, the total length of the zoom lens is caused to increasegreatly.

(ii) Letting radii of curvature of lens surfaces on the object side andon the image side of the positive lens disposed closest to the imageside included in the third lens unit be denoted by R_(31r) and R_(32r),respectively, and focal lengths of the positive lens and the third lensunit be denoted by f₃₂ and f₃, respectively, the following conditionsare satisfied:

1.0<|R _(31r) /R _(32r)|<5.0  (2)

1.5<f ₃ /f ₃₂<5.0  (3).

The inequalities of conditions (2) and (3) both have an aim to regulatethe curvature of the lens surface closest to the image side in the thirdlens unit. When the lower limits are exceeded, as this means that thecurvature of the rear surface and the focal length of the positive lensare too loose, it becomes difficult to keep the back focal distance longenough or accomplish the object of the invention. When the upper limitsare exceeded, it becomes difficult to correct the spherical aberrationsof higher orders which are produced when the light beam emerging fromthe third lens unit enters the fourth lens unit having the focusingfunction. So, the improvement of the performance cannot be achieved.

(iii) Letting the sum of air separations between the first and secondlens units and between the second and third lens units be denoted by L,a semiangle of field in the wide-angle end be denoted by ω, focallengths in the wide-angle end and in the telephoto end of the zoom lensbe denoted by f_(W) and f_(T), respectively, a focal length of thefourth lens unit be denoted by f₄, and an air separation between thethird and fourth lens units for an infinitely distant object in thetelephoto end be denoted by D, the following conditions are satisfied:

0.66<L/(f _(T)·tan ω)<1.17  (4)

4.00<f ₄ /f _(W)<7.00  (5)

0.10<D/f _(T)<0.30  (6).

The inequalities of condition (4) are to optimize the relationshipbetween the space for zooming movement of the second lens unit and thezoom ratio. When the upper limit is exceeded, as this means that thespace for zooming movement is too wide, the total length of the entiresystem is caused to increase greatly. When the lower limit is exceeded,the negative refractive power of the second lens unit has to bestrengthened. Otherwise, the share of varying the focal length could notbe earned. So, the negative Petzval sum that represents the curvature offield is increased objectionably.

The inequalities of condition (5) are to optimize the back focaldistance. When the upper limit is exceeded, the back focal distancebecomes longer than necessary, causing the total length of the entiresystem to increase greatly. When the lower limit is exceeded, it becomesdifficult to secure the back focal distance long enough.

The inequalities of condition (6) are to optimize the relationshipbetween the space for focusing movement of the fourth lens unit and thefocal length in the telephoto end of the entire system. When the airseparation D is taken at a larger value than the upper limit, the totallength of the entire system is caused to increase objectionably. Whenthe lower limit is exceeded, it becomes impossible to secure asufficient space for the focusing purpose. So, a problem arises in themanageability of the zoom lens.

Also, letting a radius of curvature of a lens surface closest to theobject side in the third lens unit be denoted by R_(31f), a focal lengthof a lens disposed closest to the object side in the third lens unit bedenoted by f₃₁, and a focal length of the third lens unit be denoted byf₃, the following conditions are satisfied:

−0.60<R _(31f) /f ₃<−0.10  (7)

0.30<R _(31f) /f ₃₁<0.90  (8).

The inequalities of conditions (7) and (8) both have an aim to regulatethe curvature of the lens surface closest to the object side in thethird lens unit. When the upper limits are exceeded, as this means thatthe curvature and the focal length of the positive lens are too loose,it becomes difficult to keep the back focal distance long enough oraccomplish the object of the invention. When the lower limits areexceeded, it becomes difficult to correct the spherical aberrations ofhigher orders which are produced when the light beam emerging from thesecond lens unit enters the third lens unit in the wide-angle end. So,the improvement of the performance cannot be achieved.

In the meantime, the chromatic aberrations for the telephoto end must becorrected sufficiently. For this purpose, all that are necessary to formthe second lens unit are at least two negative lenses and at least onepositive lens. In the present embodiment, as described above, anadditional negative lens is used as disposed closest to the image sidein the second lens unit to thereby widen the interval between theprincipal points of the second and third lens units. This negative lenscontributes to even more increase of the back focal distance.

To achieve further improvements of the correction of aberrations,particularly chromatic aberrations, it is preferable to use at least onecemented lens in the third lens unit as shown in the numerical examples1 to 3 and 9. As mentioned above, the image quality of the video camerais ever increasing. Therefore, the chromatic aberrations, particularlythe lateral chromatic aberration, which little mattered in the past,come to be a problem. So, this is corrected well.

Also, in the present embodiment, in order that the image of the firstlens unit be made small, the aperture stop is disposed just in front ofthe third lens unit. However, the location of the aperture stop is notconfined to this position. It is to be understood that the aperture stopmay otherwise put to either the space between the third and fourth lensunits or the space between the negative and positive lenses in the thirdlens unit.

Incidentally, in the present embodiment, the third lens unit is made ofminus-plus refractive power arrangement in this order from the objectside to lengthen the exit pupil, so that the light rays emerging fromthe zoom lens become almost telecentric. As the color separation prismis positioned behind the zoom lens, the angle at which the light raysenter the color separation prism is made gentler to thereby eliminatethe change of the reflection characteristic of the color separatingsystem by the wavelengths. This permits the color separation to becarried out with high fidelity. Hence, the color reproduction of theimage is greatly improved.

Also, in the high range zoom lens such as that of the invention, thefocal length becomes very long in the telephoto end. Therefore, theperformance at or near the telephoto end is greatly influenced by thesecond lens unit. With this in mind, an aspheric surface may beintroduced into this second lens unit. If so, it is possible to increasethe optical performance.

Incidentally, the aspheric surface is fundamentally aimed at correctingspherical aberrations. So, it is desirable to adopt a shape in which thepositive refractive power becomes progressively weaker toward the marginof the lens.

Further, in order to correct aberrations, particularly chromaticaberrations, even better, at least one positive lens in the fourth lensunit is made up from a glass whose Abbe number νd falls within thefollowing range:

νd >64.0

This inequality of condition is for correcting the lateral chromaticaberration well. When the lower limit is exceeded, as this means thatthe Abbe number is too small, under-correction of the lateral chromaticaberration results. So, it should be avoided.

Next, the numerical data for the numerical examples 1 to 9 of theinvention are shown in the tables below, where Ri is the radius ofcurvature of the i-th lens surface, when counted from the object side,Di is the i-th axial thickness or air separation, when counted from theobject side, and Ni and νi are respectively the refractive index andAbbe number of the glass of the i-th lens element, when counted from theobject side.

It is to be noted that R28 and R29 in the numerical example 1, R26 toR27 in the numerical examples 2, 3 and 9 and R24 and R25 in thenumerical examples 4 to 8, each define a protection glass plate. R30 toR33 in the numerical example 1, R28 to R31 in the numerical examples 2,3 and 9 and R26 to R29 in the numerical examples 4 to 8, each define aglass block such as a color separation prism, an optical filter or aface plate.

The values of the factors in the above-described conditions (1) to (8)for the numerical examples 1 to 9 are listed in Table-1.

The shape of the aspheric surface is expressed in the coordinates withan X axis in the axial direction and an H axis in the directionperpendicular to the optical axis, the direction in which light advancesbeing taken as positive, by the following equation:$X = {\frac{\left( {1/R} \right)H^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)\quad \left( {H/R} \right)^{2}}}} + {BH}^{4} + {CH}^{6} + {DH}^{8} + {EH}^{10}}$

where R is the radius of the osculating sphere, and K, B, C, D and E arethe aspheric coefficients.

In the values of the aspheric coefficients, the notation “e-0X” means10^(−x).

TABLE 1 Condition Numerical Example No. 1 2 3 4 5 6 7 8 9 (1) 10.5910.90 11.09 9.96 9.59 10.03 10.03 10.04 9.08 (2) 2.428 1.313 1.338 3.3603.191 3.296 3.299 3.353 1.373 (3) 2.304 2.483 2.608 2.824 2.648 2.7502.771 2.893 2.880 (4) 0.935 0.932 0.908 0.917 0.925 0.921 0.921 0.9180.998 (5) 5.159 5.178 5.265 5.281 5.220 5.294 5.288 5.278 5.266 (6)0.225 0.192 0.208 0.260 0.228 0.253 0.255 0.258 0.20 (7) −0.255 −0.328−0.386 −0.269 −0.276 −0.268 −0.275 −0.257 −0.269 (8) 0.412 0.597 0.7580.598 0.548 0.575 0.594 0.593 0.750 Numerical Example 1: f = 1˜15.42 Fno = 1.65˜2.61  2ω = 59.0°˜4.2° R1 = 14.996 D1 = 0.30 N1 = 1.846660 ν1= 23.8 R2 = 7.420 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −55.755 D3 =0.04 R4 = 6.528 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 16.528 D5 =Variable R6 = 6.618 D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.639 D7 =0.67 R8 = −5.612 D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 6.435 D9 = 0.11R10 = 3.282 D10 = 0.55 N6 = 1.846660 ν6 = 23.8 R11 = −4.917 D11 = 0.09R12 = −3.110 D12 = 0.14 N7 = 1.772499 ν7 = 49.6 R13 = 52.786 D13 =Variable R14 = Stop D14 = 0.60 R15 = −4.354 D15 = 0.14 N8 = 1.772499 ν8= 49.6 R16 = 7.442 D16 = 0.44 N9 = 1.846660 ν9 = 23.8 R17 = −12.275 D17= 0.39 R18* = 12.057 D18 = 0.81 N10 = 1.583126 ν10 = 59.4 R19 = −3.212D19 = 0.17 N11 = 1.846660 ν11 = 23.8 R20 = −4.966 D20 = Variable R21 =7.169 D21 = 0.60 N12 = 1.583126 ν12 = 59.4 R22 = −10.614 D22 = 0.03 R23= 10.386 D23 = 0.18 N13 = 1.805181 ν13 = 25.4 R24 = 3.038 D24 = 0.74 N14= 1.487490 ν14 = 70.2 R25 = −68.690 D25 = 0.03 R26 = 5.180 D26 = 0.42N15 = 1.603420 ν15 = 38.0 R27 = 18.977 D27 = 0.35 R28 = ∞ D28 = 0.35 N16= 1.516330 ν16 = 64.2 R29 = ∞ D29 = 0.71 R30 = ∞ D30 = 0.24 N17 =1.550000 ν17 = 60.0 R31 = ∞ D31 = 3.54 N18 = 1.589130 ν18 = 61.2 R32 = ∞D32 = 0.42 N19 = 1.520000 ν19 = 64.0 R33 = ∞ Variable Focal LengthSeparation 1.00 6.04 15.42 D5  0.16 5.31 6.76 D13 6.89 1.75 0.30 D203.50 2.49 3.47 Aspheric Coefficients: R18: K = −1.59540e + 00 B =−2.75872e − 03 C = 2.79593e − 04 D = −1.02196e − 04 E = 1.51980e − 05Numerical Example 2: f = 1˜15.43  Fno = 1.65˜2.65  2ω = 59.0°˜4.2° R1 =14.448 D1 = 0.30 N1 = 1.846660 ν1 = 23.8 R2 = 7.307 D2 = 0.99 N2 =1.603112 ν2 = 60.7 R3 = −68.744 D3 = 0.04 R4 = 6.579 D4 = 0.57 N3 =1.696797 ν3 = 55.5 R5 = 17.338 D5 = Variable R6 = 7.309 D6 = 0.16 N4 =1.882997 ν4 = 40.8 R7 = 1.622 D7 = 0.65 R8 = −5.306 D8 = 0.14 N5 =1.882997 ν5 = 40.8 R9 = 8.431 D9 = 0.11 R10 = 3.341 D10 = 0.58 N6 =1.846660 ν6 = 23.8 R11 = −5.076 D11 = 0.06 R12 = −3.356 D12 = 0.14 N7 =1.772499 ν7 = 49.6 R13 = 25.317 D13 = Variable R14 = Stop D14 = 0.57 R15= −5.556 D15 = 0.14 N8 = 1.772499 ν8 = 49.6 R16 = 4.910 D16 = 0.42 N9 =1.846660 ν9 = 23.8 R17 = −54.213 D17 = 0.55 R18* = 7.365 D18 = 0.85 N10= 1.583126 ν10 = 59.4 R19 = −3.269 D19 = 0.17 N11 = 1.846660 ν11 = 23.8R20 = −5.610 D20 = Variable R21* = 10.245 D21 = 0.53 N12 = 1.583126 ν12= 59.4 R22 = −11.430 D22 = 0.03 R23 = 4.850 D23 = 0.18 N13 = 1.846660ν13 = 23.8 R24 = 2.832 D24 = 0.92 N14 = 1.487490 ν14 = 70.2 R25 =−11.531 D25 = 0.35 R26 = ∞ D26 = 0.35 N15 = 1.516330 ν15 = 64.2 R27 = ∞D27 = 0.71 R28 = ∞ D28 = 0.24 N16 = 1.550000 ν16 = 60.0 R29 = ∞ D29 =3.54 N17 = 1.589130 ν17 = 61.2 R30 = ∞ D30 = 0.42 N18 = 1.520000 ν18 =64.0 R31 = ∞ Variable Focal Length Separation 1.00 6.15 15.43 D5  0.165.31 6.76 D13 6.90 1.76 0.31 D20 2.99 1.95 2.96 Aspheric Coefficients:R18: K = 1.71304e + 00 B = −3.52919e − 03 C = 1.32785e − 04 D =−7.87389e − 05 E = 1.36494e − 05 R21: K = 9.97285e + 00 B = −1.90572e −03 C = −8.84882e − 05 D = 3.08971e − 05 E = −5.17678e − 06 NumericalExample 3: f = 1˜16.08  Fno = 1.65˜2.75  2ω = 59.0°˜4.0° R1 = 14.556 D1= 0.30 N1 = 1.846660 ν1 = 23.8 R2 = 7.387 D2 = 0.99 N2 = 1.603112 ν2 =60.7 R3 = −66.855 D3 = 0.04 R4 = 6.636 D4 = 0.57 N3 = 1.696797 ν3 = 55.5R5 = 17.133 D5 = Variable R6 = 7.155 D6 = 0.16 N4 = 1.882997 ν4 = 40.8R7 = 1.600 D7 = 0.65 R8 = −5.391 D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 =8.731 D9 = 0.11 R10 = 3.271 D10 = 0.58 N6 = 1.846660 ν6 = 23.8 R11 =−5.348 D11 = 0.06 R12 = −3.464 D12 = 0.14 N7 = 1.772499 ν7 = 49.6 R13 =21.334 D13 = Variable R14 = Stop D14 = 0.54 R15 = −6.457 D15 = 0.14 N8 =1.772499 ν8 = 49.6 R16 = 4.445 D16 = 0.42 N9 = 1.846660 ν9 = 23.8 R17 =60.000 D17 = 0.51 R18* = 7.173 D18 = 0.85 N10 = 1.583126 ν10 = 59.4 R19= −3.304 D19 = 0.17 N11 = 1.846660 ν11 = 23.8 R20 = −5.362 D20 =Variable R21* = 10.013 D21 = 0.53 N12 = 1.583126 ν12 = 59.4 R22 =−14.510 D22 = 0.03 R23 = 4.830 D23 = 0.18 N13 = 1.846660 ν13 = 23.8 R24= 2.832 D24 = 0.92 N14 = 1.487490 ν14 = 70.2 R25 = −10.073 D25 = 0.35R26 = ∞ D26 = 0.35 N15 = 1.516380 ν15 = 64.2 R27 = ∞ D27 = 0.71 R28 = ∞D28 = 0.24 N16 = 1.550000 ν16 = 60.0 R29 = ∞ D29 = 3.54 N17 = 1.589130ν17 = 61.2 R30 = ∞ D30 = 0.42 N18 = 1.520000 ν18 = 64.0 R31 = ∞ VariableFocal Length Separation 1.00 6.21 16.08 D5  0.16 5.41 6.89 D13 7.04 1.800.32 D20 3.36 2.28 3.35 Aspheric Coefficients: R18: K = 1.70065e + 00 B= −3.71689e − 03 C = 1.29974e − 04 D = −7.31900e − 05 E = 1.19400e − 05R21: K = 9.54122e + 00 B = −1.84461e − 03 C = −8.80745e − 05 D =2.61881e − 05 E = −4.30759e − 06 Numerical Example 4: f = 1˜16.10  Fno =1.65˜2.67  2ω = 59.0°˜4.0° R1 = 14.266 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.309 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −76.889 D3 = 0.04 R4 =6.599 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 17.367 D5 = Variable R6 =8.250 D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.567 D7 = 0.67 R8 = −6.044D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 9.892 D9 = 0.11 R10 = 3.116 D10 =0.55 N6 = 1.846660 ν6 = 23.8 R11 = −5.918 D11 = 0.05 R12 = −4.064 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 10.682 D13 = Variable R14 = Stop D14= 0.66 R15 = −3.562 D15 = 0.18 N8 = 1.772499 ν8 = 49.6 R16 = −16.115 D16= 0.32 R17* = 12.110 D17 = 0.81 N9 = 1.603112 ν9 = 60.7 R18 = −3.604 D18= Variable R19* = 5.099 D19 = 0.66 N10 = 1.583126 ν10 = 59.4 R20 =−27.004 D20 = 0.30 R21 = 12.568 D21 = 0.18 N11 = 1.846660 ν11 = 23.8 R22= 4.078 D22 = 0.78 N12 = 1.487490 ν12 = 70.2 R23 = −5.659 D23 = 0.35 R24= ∞ D24 = 0.35 N13 = 1.516330 ν13 = 64.2 R25 = ∞ D25 = 0.71 R26 = ∞ D26= 0.24 N14 = 1.550000 ν14 = 60.0 R27 = ∞ D27 = 3.54 N15 = 1.589130 ν15 =61.2 R28 = ∞ D28 = 0.42 N16 = 1.520000 ν16 = 64.0 R29 = ∞ Variable FocalLength Separation 1.00 5.98 16.10 D5  0.16 5.37 6.84 D13 7.01 1.81 0.34D18 4.16 3.15 4.19 Aspheric Coefficients: R17: K = 2.56943e + 01 B =−6.08402e − 03 C = −1.11466e − 04 D = −2.83007e − 05 E = 0.00000e + 00R19: K = 6.27497e − 01 B = −3.50832e − 03 C = 4.82436e − 05 D =−1.48759e − 05 E = 0.00000e + 00 Numerical Example 5: f = 1˜16.08  Fno =1.65˜2.68  2ω = 59.0°˜4.0° R1 = 13.491 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.108 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −156.379 D3 = 0.04 R4= 6.721 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 18.681 D5 = Variable R6 =8.425 D6 = 0.16 N4 = 1.834807 ν4 = 42.7 R7 = 1.508 D7 = 0.67 R8 = −5.965D8 = 0.14 N5 = 1.834807 ν5 = 42.7 R9 = 7.802 D9 = 0.11 R10 = 3.066 D10 =0.55 N6 = 1.846660 ν6 = 23.8 R11 = −7.878 D11 = 0.05 R12 = −4.920 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 12.676 D13 = Variable R14 = (Stop)D14 = 0.67 R15 = −3.452 D15 = 0.18 N8 = 1.696797 ν8 = 55.5 R16 = −16.552D16 = 0.28 R17* = 11.329 D17 = 0.81 N9 = 1.583126 ν9 = 59.4 R18 = −3.550D18 = Variable R19* = 4.725 D19 = 0.65 N10 = 1.583126 ν10 = 59.4 R20 =−34.150 D20 = 0.30 R21 = 13.584 D21 = 0.18 N11 = 1.846660 ν11 = 23.8 R22= 3.861 D22 = 0.78 N12 = 1.487490 ν12 = 70.2 R23 = −5.180 D23 = 0.35 R24= ∞ D24 = 0.35 N13 = 1.516330 ν13 = 64.2 R25 = ∞ D25 = 0.71 R26 = ∞ D26= 0.24 N14 = 1.550000 ν14 = 60.0 R27 = ∞ D27 = 3.54 N15 = 1.589130 ν15 =61.2 R28 = ∞ D28 = 0.42 N16 = 1.520000 ν16 = 64.0 R29 = ∞ Variable FocalLength Separation 1.00 6.07 16.08 D5  0.16 5.42 6.90 D13 7.06 1.81 0.33D18 3.63 2.63 3.67 Aspheric Coefficients: R17: K = 2.18527e + 01 B =−6.25263e − 03 C = −1.88327e − 04 D = −1.78777e − 05 E = 0.00000e + 00R19: K = 6.43103e − 01 B = −4.14656e − 03 C = 5.29237e − 05 D =−2.13505e − 05 E = 0.00000e + 00 Numerical Example 6: f = 1˜16.10  Fno =1.65˜2.67  2ω = 59.0°˜4.0° R1 = 13.746 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.190 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −111.074 D3 = 0.04 R4= 6.652 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 17.839 D5 = Variable R6 =8.785 D6 = 0.16 N4 = 1.834807 ν4 = 42.7 R7 = 1.517 D7 = 0.64 R8 = −6.121D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 8.341 D9 = 0.11 R10 = 3.088 D10 =0.55 N6 = 1.846660 ν6 = 23.8 R11 = −7.139 D11 = 0.05 R12 = −4.657 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 12.972 D13 = Variable R14 = Stop D14= 0.65 R15 = −3.626 D15 = 0.18 N8 = 1.772499 ν8 = 49.6 R16 = −14.465 D16= 0.37 R17* = 12.097 D17 = 0.81 N9 = 1.583126 ν9 = 59.4 R18 = −3.671 D18= Variable R19* = 4.970 D19 = 0.64 N10 = 1.583126 ν10 = 59.4 R20 =−50.896 D20 = 0.30 R21 = 10.878 D21 = 0.18 N11 = 1.846660 ν11 = 23.8 R22= 3.860 D22 = 0.80 N12 = 1.487490 ν12 = 70.2 R23 = −5.570 D23 = 0.35 R24= ∞ D24 = 0.35 N13 = 1.516330 ν13 = 64.2 R25 = ∞ D25 = 0.71 R26 = ∞ D26= 0.24 N14 = 1.550000 ν14 = 60.0 R27 = ∞ D27 = 3.54 N15 = 1.589130 ν15 =61.2 R28 = ∞ D28 = 0.42 N16 = 1.520000 ν16 = 64.0 R29 = ∞ Variable FocalLength Separation 1.00 6.00 16.10 D5  0.17 5.40 6.88 D13 7.05 1.81 0.33D18 4.03 3.03 4.07 Aspheric Coefficients: R17: K = 2.27903e + 01 B =−5.62838e − 03 C = −1.18773e − 04 D = −1.36886e − 05 E = 0.00000e + 00R19: K = 7.41373e − 01 B = −3.61778e − 03 C = 1.31439e − 05 D =−1.23548e − 05 E = 0.00000e + 00 Numerical Example 7: f = 1˜16.10  Fno =1.65˜2.68  2ω = 59.0°˜4.0° R1 = 13.567 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.118 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −125.995 D3 = 0.01 R4= 6.654 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 18.010 D5 = Variable R6 =8.678 D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.526 D7 = 0.64 R8 = −5.907D8 = 0.14 N5 = 1.834807 ν5 = 42.7 R9 = 7.868 D9 = 0.11 R10 = 3.102 D10 =0.55 N6 = 1.846660 ν6 = 23.8 R11 = −6.652 D11 = 0.04 R12 = −4.703 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 13.795 D13 = Variable R14 = Stop D14= 0.65 R15 = −3.705 D15 = 0.18 N8 = 1.772499 ν8 = 49.6 R16 = −16.327 D16= 0.35 R17* = 11.960 D17 = 0.81 N9 = 1.583126 ν9 = 59.4 R18 = −3.626 D18= Variable R19* = 5.020 D19 = 0.64 N10 = 1.583126 ν10 = 59.4 R20 =−54.719 D20 = 0.30 R21 = 10.668 D21 = 0.18 N11 = 1.846660 ν11 = 23.8 R22= 3.837 D22 = 0.81 N12 = 1.487490 ν12 = 70.2 R23 = −5.488 D23 = 0.35 R24= ∞ D24 = 0.35 N13 = 1.516330 ν13 = 64.2 R25 = ∞ D25 = 0.71 R26 = ∞ D26= 0.24 N14 = 1.550000 ν14 = 60.0 R27 = ∞ D27 = 3.54 N15 = 1.589130 ν15 =61.2 R28 = ∞ D28 = 0.42 N16 = 1.520000 ν16 = 64.0 R29 = ∞ Variable FocalLength Separation 1.00 5.99 16.10 D5  0.17 5.42 6.90 D13 7.05 1.81 0.33D18 4.07 3.07 4.11 Aspheric Coefficients: R17: K = 2.23235e + 01 B =−5.82580e − 03 C = −1.05814e − 04 D = −1.52391e − 05 E = 0.00000e + 00R19: K = 7.55412e − 01 B = −3.56320e − 03 C = 1.14992e − 05 D =−1.20454e − 05 E = 0.00000e + 00 Numerical Example 8: f = 1˜16.10  Fno =1.65˜2.68  2ω = 59.0°˜4.0° R1 = 14.274 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.309 D2 = 1.01 N2 = 1.603112 ν2 = 60.7 R3 = −76.524 D3 = 0.04 R4 =6.599 D4 = 0.58 N3 = 1.696797 ν3 = 55.5 R5 = 17.369 D5 = Variable R6 =8.182 D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.559 D7 = 0.67 R8 = −6.012D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 10.283 D9 = 0.11 R10 = 3.100 D10= 0.55 N6 = 1.846660 ν6 = 23.8 R11 = −6.133 D11 = 0.05 R12 = −4.154 D12= 0.14 N7 = 1.772499 ν7 = 49.6 R13 = 10.551 D13 = Variable R14 = StopD14 = 0.66 R15 = −3.512 D15 = 0.18 N8 = 1.772499 ν8 = 49.6 R16 = −15.411D16 = 0.32 R17* = 11.773 D17 = 0.81 N9 = 1.583126 ν9 = 59.4 R18 = −3.512D18 = Variable R19* = 5.060 D19 = 0.67 N10 = 1.583126 ν10 = 59.4 R20 =−25.181 D20 = 0.30 R21 = 13.185 D21 = 0.18 N11 = 1.846660 ν11 = 23.8 R22= 4.105 D22 = 0.78 N12 = 1.487490 ν12 = 70.2 R23 = −5.634 D23 = 0.35 R24= ∞ D24 = 0.35 N13 = 1.516330 ν13 = 64.2 R25 = ∞ D25 = 0.71 R26 = ∞ D26= 0.24 N14 = 1.550000 ν14 = 60.0 R27 = ∞ D27 = 3.54 N15 = 1.589130 ν15 =61.2 R28 = ∞ D28 = 0.42 N16 = 1.520000 ν16 = 64.0 R29 = ∞ Variable FocalLength Separation 1.00 6.00 16.10 D5  0.16 5.37 6.84 D13 7.02 1.81 0.34D18 4.11 3.11 4.16 Aspheric Coefficients: R17: K = 2.24174e + 01 B =−6.23144e − 03 C = −1.05546e − 04 D = −1.73115e − 05 E = 0.00000e + 00R19: K = 5.10561e − 01 B = −3.44827e − 03 C = 4.58683e − 05 D =−1.23288e − 05 E = 0.00000e + 00 Numerical Example 9: f = 1˜14.61  Fno =1.65˜2.53  2ω = 59.0°˜4.4° R1 = 14.769 D1 = 0.30 N1 = 1.846660 ν1 = 23.8R2 = 7.439 D2 = 0.99 N2 = 1.603112 ν2 = 60.7 R3 = −75.427 D3 = 0.04 R4 =6.741 D4 = 0.57 N3 = 1.696797 ν3 = 55.5 R5 = 17.783 D5 = Variable R6 =6.595 D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.630 D7 = 0.65 R8 = −4.919D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 7.350 D9 = 0.11 R10 = 3.412 D10 =0.58 N6 = 1.846660 ν6 = 23.8 R11 = −4.519 D11 = 0.06 R12 = −3.099 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 51.053 D13 = Variable R14 = (Stop)D14 = 0.55 R15 = −5.958 D15 = 0.14 N8 = 1.772499 ν8 = 49.6 R16 = 4.450D16 = 0.42 N9 = 1.846660 ν9 = 23.8 R17 = 67.055 D17 = 0.34 R18* = 6.761D18 = 0.85 N10 = 1.583126 ν10 = 59.4 R19 = −3.017 D19 = 0.17 N11 =1.846660 ν11 = 23.8 R20 = −4.923 D20 = Variable R21* = 10.622 D21 = 0.53N12 = 1.516330 ν12 = 64.2 R22 = −12.034 D22 = 0.03 R23 = 4.856 D23 =0.18 N13 = 1.846660 ν13 = 23.8 R24 = 2.832 D24 = 0.92 N14 = 1.516330 ν14= 64.2 R25 = −11.264 D25 = Variable R26* = −5.117 D26 = 0.35 N15 =1.516330 ν15 = 64.2 R27 = −4.790 D27 = 0.71 R28 = ∞ D28 = 0.24 N16 =1.550000 ν16 = 60.0 R29 = ∞ D29 = 3.54 N17 = 1.589130 ν17 = 61.2 R30 = ∞D30 = 0.42 N18 = 1.520000 ν18 = 64.0 R31 = ∞ Variable Focal LengthSeparation 1.00 6.03 14.61 D5  0.16 5.41 6.89 D13 7.03 1.78 0.30 D203.46 2.26 3.03 D25 0.71 1.91 1.14 Aspheric Coefficients: R18: K =1.42924e + 00 B = −3.77394e − 03 C = 8.07351e − 05 D = −4.17413e − 05 E= 8.03871e − 06 R21: K = 1.02571e + 01 B = −1.93447e − 03 C = −8.46904e− 05 D = 2.46044e − 05 E = −3.54895e − 06 R26: K = 2.07625e − 01 B =−3.08758e − 04 C = 1.00476e − 04 D = 1.13951e − 05 E = −1.14348e − 05

According to the invention, in the foregoing embodiment thereof, itbecomes possible to provide a zoom lens of the rear focus type whoserange is as high as more than 15 and whose relative aperture is as largeas 1.6 in the F-number, while still permitting the back focal distanceto be secured large enough to accommodate an optical element such as thecolor separation prism or an optical element which is aimed to protectthe zoom lens. In addition, the optical performance is maintained stablethroughout the entire zooming range and throughout the entire focusingrange. Using such an zoom lens, a compact, light weight, highperformance video camera of the lens add-on type can be realized.

Another embodiment of the invention in which further improvements aremade will be described below.

A zoom lens of the rear focus type according to the present embodimentof the invention comprises, in order from an object side, a first lensunit of positive refractive power, a second lens unit of negativerefractive power, a third lens unit of positive refractive power and afourth lens unit of positive refractive power, the second lens unitbeing moved axially toward the image side to vary the focal length fromthe wide-angle end to the telephoto end, and the fourth lens units beingmoved axially, while depicting a locus convex toward the object side, tocompensate for the image shift with zooming. Focusing is performed bymoving the fourth lens unit. The fourth lens unit includes a positivefirst lens, a negative second lens and a positive third lens and has atleast one aspheric surface. Letting a focal length of the third lensunit be denoted by f₃, focal lengths in the wide-angle end and in thetelephoto end of the zoom lens be denoted by f_(W) and f_(T),respectively, and an F-number in the wide-angle end of the zoom lens bedenoted by F_(NW), and putting

f _(M)={square root over (f _(W) ·f _(T)+L )},

the following condition is satisfied:

3.44<f ₃ ×F _(NW) /f _(M)<15.38  (9).

FIGS. 20 to 22 are longitudinal section views of the respectivenumerical examples 10 to 12 of zoom lenses of the rear focus typeaccording to the present embodiment of the invention. The aberrations ofthe numerical example 10 are shown in FIGS. 23-1 to 23-4 through FIGS.25-1 to 25-4. The aberrations of the numerical example 11 are shown inFIGS. 26-1 to 26-4 through FIGS. 28-1 to 28-4. The aberrations of thenumerical example 12 are shown in FIGS. 29-1 to 29-4 through FIGS. 31-1to 31-4. Of the aberration curves, the ones of FIGS. 23-1 to 23-4, FIGS.26-1 to 26-4 and FIGS. 29-1 to 29-4 are in the wide-angle end, the onesof FIGS. 24-1 to 24-4, FIGS. 27-1 to 27-4 and FIGS. 30-1 to 30-4 are ina middle focal length position, and the ones of FIGS. 25-1 to 25-4,FIGS. 28-1 to 28-4 and FIGS. 31-1 to 31-4 are in the telephoto end.

In the lens block diagrams, reference character L1 denotes a first lensunit of positive refractive power, reference character L2 denotes asecond lens unit of negative refractive power, reference character L3denotes a third lens unit of positive refractive power and referencecharacter L4 denotes a fourth lens unit of positive refractive power. Anaperture stop SP is disposed in front of the third lens unit L3.Reference character G denotes a glass block such as a color separationoptical system, a face plate or a filter. Reference character IP denotesan image plane.

In the present embodiment, during zooming from the wide-angle end to thetelephoto end, the second lens unit moves toward the image side asindicated by the arrow, while simultaneously axially moving the fourthlens unit in a locus convex toward the object side to compensate for theshift of the image plane.

For focusing purposes, the fourth lens unit moves axially. That is, therear focusing method is employed. When focusing on an object at infinityor at a minimum distance, the fourth lens unit moves in a locus shown inFIG. 20 by a solid line curve 4 a or a dashed line curve 4 b,respectively, to compensate for the image shift with zooming. It isnoted that the first and third lens units remain stationary duringzooming and during focusing.

In the present embodiment, the provision for compensating for the imageshift with zooming and for focusing is made on the common or fourth lensunit. In particular, as shown by the curves 4 a and 4 b in FIG. 20, alocus of movement of the fourth lens unit with zooming from thewide-angle end to the telephoto end is made convex toward the objectside. This assures efficient utilization of the air space between thethird and fourth lens units. The shortening of the total length of theentire system is thus achieved advantageously.

In the present embodiment, with the setting, for example, in thetelephoto end, when focusing is performed to suit from an infinitelydistant object to closer objects, the fourth lens unit moves toward theobject side as shown by a straight line 4 c in FIG. 20.

According to the invention, the fourth lens unit is constructed with, inorder from an object side, a positive first lens of bi-convex form, anegative second lens of either bi-concave form or meniscus form convextoward the object side, and a positive third lens of bi-convex form,totaling three lenses, and is characterized by satisfying the abovecondition (9).

By using three lenses in constructing the fourth lens unit as describedabove, the exit pupil is put to so long a position (distance) that thelight beam emerging from the lens system becomes telecentric. As thecolor separation prism is disposed behind the lens system, the angle atwhich the light rays enter the color separation prism is made gentler.Under such a condition, color separation is carried out effectively, sothat the color reproduction of the image is thus enhanced.

Then, in order to correct the aberrations well when increasing therelative aperture and the range of variation of the focal length, whilekeeping hold of the back focal distance at a predetermined value, therefractive power (focal length f₃) of the third lens unit and theF-number are determined so as to satisfy the condition (9).

The condition (9) regulates the focal length f₃ of the third lens unit,and is thus greatly related to the back focal distance.

When the lower limit of the condition (9) is exceeded, as this impliesthat the F-number F_(NW) in the wide-angle end is too bright, or thatthe focal length of the third lens unit is too short, it becomesdifficult to secure the predetermined value of the back focal distance.Conversely, when the focal length of the third lens unit is made toomuch longer than the upper limit, or when the F-number F_(NW) in thewide-angle end is too much darker, the back focal distance becomeslonger, but the distance between the third lens unit and the fourth lensunit is caused to increase, which in turn causes the total length of theentire lens system to become longer. Therefore, it becomes difficult toimprove the compact form.

Other characteristic features of the zoom lens of the rear focus typeaccording to the present embodiment of the invention are describedbelow.

One of the aims of the invention is to extend the zooming range. To thisend, it is desirable that the chromatic aberrations that arise fromzooming are to be cancelled out in the first and second lens units.Contradictorily to this, the ways the first and second lens unitsproduce lateral chromatic aberration with zooming are greatly differentfrom each other. So, in the wide-angle end, it is liable to incline toover-correction. Therefore, the fourth lens unit is under-corrected forlateral chromatic aberration, so that the chromatic aberrations are keptin balance throughout the extended zooming range.

In this case, for the longitudinal chromatic aberration, when the zoomratio is small, correction can be done without causing its balance tocollapse greatly.

Therefore, it is also possible to make the third lens unit in the formof a positive single lens. However, to aim at much higher a range andmuch larger a relative aperture simultaneously, as in the invention, thelongitudinal chromatic aberration is under-corrected as a whole. So, itbecomes difficult to maintain a good stability of optical performance athigh level.

On this account, according to the invention, the third lens unit isconstructed with a negative first lens, a positive second lens ofmeniscus form convex toward the image side and at least one cemented orthird lens, thereby correcting chromatic aberrations at optimumthroughout the extended zooming range. Also, the spherical aberrationwhich has flare components of higher orders is suppressed to a minimum.

In particular, the third lens unit is constructed with a negative firstlens either of bi-concave form or of meniscus form convex toward theobject side, a positive second lens of meniscus form convex toward theimage side, a positive third lens of bi-convex form and a negativefourth lens of meniscus form convex toward the image side. Of these, thethird lens and the fourth lens are cemented together to form thecemented lens.

It will be appreciated that, in the invention, the form and theconstruction and arrangement of the constituent lenses are simple.Nonetheless, the zooming range is increased to as high as 15.5, and therelative aperture is as large as 1.45 to 1.65 in F-number in thewide-angle end. Moreover, high optical performance is maintained stablethroughout.

Fundamentally, the cemented form may be adopted in each lens unit. Ifso, intra-unit decentering can be suppressed effectively, and it ispossible to assure uniformity of the quality of the manufacturedproducts. However, this lowers the degree of freedom of the design byone. Therefore, it becomes difficult to attain fulfillment of theinitial performance in such a manner that the requirements for the largerelative aperture and the compact form of the zoom lens are satisfied.

Accordingly, in the present embodiment, the third lens unit isconstructed as described above. Moreover, in the numerical example 10,the convex surface of the strongest refracting power in the third lensunit is formed to aspheric shape such that the positive refractive powerbecomes progressively weaker toward the marginal zone of the lens,thereby correcting the spherical aberration in the flare components ofhigher orders. At the same time, the intra-unit decentering issuppressed advantageously. For the zoom lens of even higher precisionaccuracy, the increase of the relative aperture is thus achieved. Also,in the numerical examples 10 and 12 shown in FIGS. 20 and 22, the fourthlens unit is constructed with inclusion of a cemented lens. As in thethird lens unit, there is produced an advantage of suppressing theintra-unit decentering. A zoom lens of even higher precision accuracy isthus achieved.

Further, in the present embodiment, the convex surface of the strongestrefracting power in the fourth lens unit is formed to aspheric shapesuch that the positive refractive power becomes progressively weakertoward the marginal zone of the lens, thereby correcting the sphericalaberration and the astigmatism. A zoom lens of even higher precisionaccuracy is thus attained while maintaining the large relative apertureand the ultra high zoom ratio.

Further, letting focal lengths in the wide-angle end and in thetelephoto end of the zoom lens be denoted by f_(W) and f_(T),respectively, and overall focal lengths of the first lens unit to thethird lens unit in the wide-angle end and in the telephoto end bedenoted by f_(MW) or f_(MT), respectively, and putting

f _(M)={square root over (f _(W) ·f _(T)+L )}

f _(AM)={square root over (f _(MW) ·f _(MT)+L )},

the following condition is satisfied:

0<f _(M) /F _(AM)<1.0  (10).

The factor in the condition (10) represents the degree of convergence ofthe light beam emerging from the third lens unit. In general, the lightbeam diverges in passing through the varifocal unit. The method ofcorrecting aberrations for most stability is, therefore, that such adiverged light beam is to be made afocal by the third lens unit.However, if the light beam emerging from the third lens unit is nearlyparallel, it would become difficult to shorten the total length of theentire lens system. On this account, the invention sets forth thecondition (10). When the condition (10) is satisfied, the third lensunit produces a converging light beam, thus assuring further shorteningof the total length of the entire system.

The significance of the inequalities of condition (10) is explainedbelow.

When the lower limit is exceeded, as this means that the light beambecomes divergent, the total length of the lens system becomes longer.In addition, the height of incidence of light on the fourth lens unitbecomes higher. Therefore, the bulk and size of the fourth lens unitincrease objectionably. When the upper limit is exceeded, as this meansthat the degree of convergence is too large, it favors improvements ofthe compact form, but the variation of the aberrations with zooming andfocusing is caused to increase. It becomes difficult to do goodcorrection of aberrations over the entire zooming range.

It is noted that, in the embodiment, the upper limit of the condition(10) may be altered to

0<f _(M) /f _(AM)<0.3  (10a)

If so, it becomes easier to make good compromise between therequirements of stabilizing the aberration correction and of shorteningthe total length of the lens system.

Also, letting the Abbe number of the glass of at least two positivelenses in the fourth lens unit be denoted by νd, the following conditionis satisfied:

νd>66.5  (11).

The inequality of condition (11) has an aim to correct the variation ofchromatic aberrations well, particularly, lateral chromatic aberrations,with zooming. When the condition (11) is violated, the lateral chromaticaberration becomes under-corrected objectionably.

Next, numerical examples 10 to 12 of the invention are shown. Thenumerical data for the examples 10 to 12 are listed in the tables, whereRi is the radius of curvature of the i-th lens surface, when countedfrom the first conjugate point, Di is the i-th axial thickness or airseparation, when counted from the first conjugate point, and Ni and νiare respectively the refractive index and Abbe number of the glass ofthe i-th lens element, when counted from the first conjugate point.

It is noted that the last eight surfaces R27 to R34 in the numericalexamples 10 and 12, or R28 to R35 in the numerical example 11, define aglass block such as a color separation optical system, a face plate or afilter.

The shape of the aspheric surface is expressed in the coordinates withan X axis in the axial direction and a Y axis in the directionperpendicular to the optical axis, the direction in which light advancesbeing taken as positive, by the following equation:$X = {\frac{\left( {1/R} \right)Y^{2}}{1 + \sqrt{1 - {\left( {1 + K} \right)\quad \left( {Y/R} \right)^{2}}}} + {BY}^{4} + {CY}^{6} + {DY}^{8} + {EY}^{10}}$

where R is the radius of the osculating sphere, and K, B, C, D and E arethe aspheric coefficients.

In the values of the aspheric coefficients, the notation “e-0X” means10^(−x).

The values of the factors in the above-described conditions (9) to (11)for the numerical examples 10 to 12 are listed in Table-2.

Numerical Example 10: f = 1˜15.41  Fno = 1.65˜2.65  2ω= 55.9°˜4.0° R1 =14.679  D1 = 0.30 N1 = 1.846660 ν1 = 23.8 R2 = 7.315  D2 = 0.94 N2 =1.603112 ν2 = 60.7 R3 = −59.942  D3 = 0.04 R4 = 6.825  D4 = 0.58 N3 =1.696797 ν3 = 55.5 R5 = 20.017  D5 = Variable R6 = 14.839  D6 = 0.16 N4= 1.882997 ν4 = 40.8 R7 = 1.570  D7 = 0.56 R8 = −5.096  D8 = 0.14 N5 =1.882997 ν5 = 40.8 R9 = 7.898  D9 = 0.11 R10 = 3.394 D10 = 0.55 N6 =1.846660 ν6 = 23.8 R11 = −5.130 D11 = 0.02 R12 = −4.356 D12 = 0.14 N7 =1.772499 ν7 = 49.6 R13 = 51.492 D13 = Variable R14 = Stop D14 = 0.45 R15= −14.996 D15 = 0.18 N8 = 1.804000 ν8 = 46.6 R16 = 4.405 D16 = 0.30 R17= −13.173 D17 = 0.35 N9 = 1.603420 ν9 = 38.0 R18 = −6.245 D18 = 0.03R19* = 4.034 D19 = 1.08 N10 = 1.583126 ν10 = 59.4 R20 = −3.337 D20 =0.18 N11 = 1.696797 ν11 = 55.5 R21 = −5.032 D21 = Variable R22 = 3.666D22 = 0.71 N12 = 1.487490 ν12 = 70.2 R23 = −51.198 D23 = 0.09 R24 =−9.824 D24 = 0.18 N13 = 1.846660 ν13 = 23.8 R25 = 13.203 D25 = 0.62 N14= 1.487490 ν14 = 70.2 R26* = −3.269 D26 = 0.35 R27 = ∞ D27 = 0.35 N15 =1.516330 ν15 = 64.2 R28 = ∞ D28 = 0.71 R29 = ∞ D29 = 0.14 N16 = 1.550000ν16 = 60.0 R30 = ∞ D30 = 0.28 N17 = 1.520000 ν17 = 69.0 R31 = ∞ D31 =3.54 N18 = 1.589130 ν18 = 61.2 R32 = ∞ D32 = 0.10 N19 = 1.550000 ν19 =60.0 R33 = ∞ D33 = 0.14 N20 = 1.516330 ν20 = 64.2 R34 = ∞ Focal LengthVariable W M T Separation 1.00 7.25 15.41 D5  0.17 5.71 6.76 D13 6.891.35 0.30 D21 3.53 2.55 3.50 Aspheric Coefficients: R19: K = 2.178e − 01B = −2.194e − 03 C = −2.367e − 04 D = 4.520e − 05 E = −8.647e − 06 R26:K = −4.680e + 00 B = −5.845e − 03 C = 1.788e − 03 D = −2.926e − 04 E =3.446e − 05 Numerical Example 11: f = 1˜15.53  Fno = 1.65˜2.65  2ω =55.9°˜3.9° R1 = 13.255  D1 = 0.30 N1 = 1.846660 ν1 = 23.8 R2 = 6.913  D2= 0.92 N2 = 1.603112 ν2 = 60.7 R3 = −360.605  D3 = 0.04 R4 = 6.814  D4 =0.58 N3 = 1.712995 ν3 = 53.8 R5 = 20.071  D5 = Variable R6 = 11.811  D8= 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.523  D7 = 0.62 R8 = −4.581  D8 =0.14 N5 = 1.882997 ν5 = 40.8 R9 = 10.887  D9 = 0.14 R10 = 3.642 D10 =0.48 N6 = 1.846660 ν6 = 23.8 R11 = −6.095 D11 = 0.01 R12 = −6.929 D12 =0.14 N7 = 1.772499 ν7 = 49.6 R13 = 18.541 D13 = Variable R14 = Stop D14= 0.40 R15 = 46.905 D15 = 0.18 N8 = 1.603112 ν8 = 60.7 R16 = 3.987 D16 =0.44 R17 = −5.638 D17 = 0.27 N9 = 1.603420 ν9 = 38.0 R18 = −4.303 D18 =0.04 R19 = 4.653 D19 = 0.87 N10 = 1.603420 ν10 = 38.0 R20 = −4.356 D20 =0.18 N11 = 1.805181 ν11 = 25.4 R21 = −9.396 D21 = Variable R22 = 4.019D22 = 0.81 N12 = 1.487490 ν12 = 70.2 R23 = −6.241 D23 = 0.03 R24 =−14.229 D24 = 0.18 N13 = 1.846660 ν13 = 23.8 R25 = 8.079 D25 = 0.04 R26= 7.167 D26 = 0.65 N14 = 1.487490 ν14 = 70.2 R27* = −4.952 D27 = 0.35R28 = ∞ D28 = 0.35 N15 = 1.516330 ν15 = 64.2 R29 = ∞ D29 = 0.71 R30 = ∞D30 = 0.14 N16 = 1.550000 ν16 = 60.0 R31 = ∞ D31 = 0.28 N17 = 1.520000ν17 = 69.0 R32 = ∞ D32 = 3.54 N18 = 1.589130 ν18 = 61.2 R33 = ∞ D33 =0.10 N19 = 1.550000 ν19 = 60.0 R34 = ∞ D34 = 0.14 N20 = 1.516330 ν20 =64.2 R35 = ∞ Focal Length Variable W M T Separation 1.00 7.46 15.53 D5 0.18 5.86 6.94 D13 7.06 1.38 0.30 D21 3.13 2.18 3.15 AsphericCoefficients: R27: K = −1.142e + 01 B = −2.587e − 03 C = 1.357e − 03 D =2.224e − 05 E = −2.530e − 05 Numerical Example 12: f = 1˜15.56  Fno =1.45˜2.35  2ω = 55.9°˜3.9° R1 = 16.744  D1 = 0.34 N1 = 1.846660 ν1 =23.8 R2 = 8.080  D2 = 1.04 N2 = 1.603112 ν2 = 60.7 R3 = −52.675  D3 =0.04 R4 = 5.788  D4 = 0.65 N3 = 1.712995 ν3 = 53.8 R5 = 16.145  D5 =Variable R6 = 11.275  D6 = 0.16 N4 = 1.882997 ν4 = 40.8 R7 = 1.652  D7 =0.69 R8 = −4.456  D8 = 0.14 N5 = 1.882997 ν5 = 40.8 R9 = 23.809  D9 =0.11 R10 = 3.944 D10 = 0.50 N6 = 1.846660 ν6 = 23.8 R11 = −6.752 D11 =0.01 R12 = −6.014 D12 = 0.12 N7 = 1.774299 ν7 = 49.6 R13 = 22.181 D13 =Variable R14 = Stop D14 = 0.35 R15 = −75.499 D15 = 0.16 N8 = 1.603112 ν8= 60.7 R16 = 3.992 D16 = 0.42 R17 = −11.584 D17 = 0.32 N9 = 1.603420 ν9= 38.0 R18 = −5.605 D18 = 0.04 R19 = 4.523 D19 = 1.06 N10 = 1.603420 ν10= 38.0 R20 = −4.495 D20 = 0.16 N11 = 1.834807 ν11 = 42.7 R21 = −12.558D21 = Variable R22 = 3.954 D22 = 0.87 N12 = 1.487490 ν12 = 70.2 R23 =−13.219 D23 = 0.04 R24 = 7.326 D24 = 0.16 N13 = 1.846660 ν13 = 23.8 R25= 2.531 D25 = 1.06 N14 = 1.487490 ν14 = 70.2 R26 = −6.095 D26 = 0.35 R27= ∞ D27 = 0.35 N15 = 1.516330 ν15 = 64.2 R28 = ∞ D28 = 0.53 R29 = ∞ D29= 0.14 N16 = 1.550000 ν16 = 60.0 R30 = ∞ D30 = 0.28 N17 = 1.520000 ν17 =69.0 R31 = ∞ D31 = 3.54 N18 = 1.589130 ν18 = 61.2 R32 = ∞ D32 = 0.10 N19= 1.550000 ν19 = 60.0 R33 = ∞ D33 = 0.14 N20 = 1.516330 ν20 = 64.2 R34 =∞ Focal Length Variable W M T Separation 1.00 7.87 15.56 D5  0.17 6.177.32 D13 7.43 1.43 0.28 D21 2.16 1.22 2.20 Aspheric Coefficients: R26: K= −8.511e + 00 B = 1.358e − 03 C = −3.565e − 04 D = 3.843e − 04 E =−4.533e − 05

TABLE 2 Condition Numerical Example No. 10 11 12  (9) 3.82 4.72 5.30(10) 0.18 0.06 0.013 (11) 70.2 70.2 70.2

According to the invention, as applied to the 4-unit form of the zoomlens of the rear focus type, the proper rules of design for the lensunits are set forth. It is, therefore, made possible to achieve a largerelative aperture, high range zoom lens which is corrected well forperformance throughout the entire extended zooming range and throughoutthe entire focusing range.

What is claimed is:
 1. A zoom lens comprising, in order from an objectside to an image side, a first lens unit having a positive refractivepower, a second lens unit having a negative refractive power, a thirdlens unit having a positive refractive power, and a fourth lens unithaving a positive refractive power, said second lens unit and saidfourth lens unit being moved to effect zooming, wherein said third lensunit consists of a negative lens component and a positive lens componentdisposed closest to the image side, and a lens surface on the image sideof said positive lens component has a refractive power which is strongerthan that of a lens surface on the object side of said positive lenscomponent, and wherein said zoom lens satisfies the following condition:8.1<BF×{square root over (F_(NM)+L )}/(f _(W)×tan ω)<13.3 where BF is adistance, when reduced to air, from a last lens surface of said zoomlens to an image plane in a wide-angle end with an object at infinity,and f_(w), F_(NW) and ω are a focal length, a full aperture F-number anda semiangle of field in the wide-angle end of said zoom lens,respectively.
 2. A zoom lens according to claim 1, satisfying thefollowing conditions: 1.0<|R _(31r) /R _(32r)|<5.0 1.5<f ₃ /f ₃₂<5.0where R_(31r) and R_(32r) are radii of curvature of lens surfaces on theobject side and on the image side of said positive lens component,respectively, and f₃₂ and f₃ are focal lengths of said positive lenscomponent and said third lens unit, respectively.
 3. A zoom lensaccording to claim 1, wherein said positive lens component of said thirdlens unit is a cemented lens consisting of a lens of positive refractivepower and a lens of negative refractive power.
 4. A zoom lens accordingto claim 1, wherein said fourth lens unit is moved to effect focusing.5. A zoom lens comprising, in order from an object side to an imageside, a first lens unit having a positive refractive power, a secondlens unit having a negative refractive power, a third lens unit having apositive refractive power and a fourth lens unit having a positiverefractive power, said second lens unit and said fourth lens unit beingmoved to effect zooming, and said third lens unit being designed not tomove for zooming, wherein said third lens unit has a negative lenscomponent disposed closest to the object side, and a lens surface on theobject side of said negative lens component has a refractive power whichis stronger than that of a lens surface on the image side of saidnegative lens component, and the following condition is satisfied:8.1<BF×{square root over (F_(NM)+L )}/(f _(W)×tan ω)<13.3 where BF is adistance, when reduced to air, from a last lens surface of said zoomlens to an image plane in a wide-angle end with an object at infinity,and f_(w), F_(NW) and ω are a focal length, a full aperture F-number anda semiangle of field in the wide-angle end of said zoom lens,respectively.
 6. A zoom lens according to claim 5, satisfying thefollowing conditions: −0.60<R _(31f) /f ₃<−0.10 0.30<R _(31f) /f ₃₁<0.90where R_(31f) is a radius of curvature of a lens surface on the objectside of said negative lens component, and f₃₁ and f₃ are focal lengthsof said negative lens component and said third lens unit, respectively.7. A zoom lens according to claim 5, wherein said third lens unitcomprises a cemented lens of positive refractive power.
 8. A zoom lensaccording to claim 5, wherein said fourth lens unit is moved to effectfocusing.